Wavelength up-conversion transparent glass ceramics

ABSTRACT

A high efficiency wavelength up-conversion transparent glass ceramics composition is provided containing a rare earth ion, which can be applied to short wavelength solid lasers, full color displays, infrared light detecting sensors, etc. The ceramic composition has the following chemical composition, in which fluoride fine crystals containing rare earth ions are preferentially precipitated: 
     
         ______________________________________                                    
 
    
     SiO 2            10-60 mol %                                           
AlO 1 .5         0-40 mol %                                            
GaO 1 .5         0-40 mol %                                            
PbF 2            5-60 mol %                                            
CdF 2            0-60 mol %                                            
GeO 2            0-30 mol %                                            
TiO 2            0-10 mol %                                            
ZrO 2            0-10 mol %                                            
ReF 3  or ReO 1 .5                                                  
                    0.05-30 mol %                                         
(Re = Er, Tm, Ho, Yb, Pr, etc.).                                          
______________________________________

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high efficiency wavelength up-conversiontransparent glass ceramics composition containing rare earth ions, whichcan be applied to short wavelength solid lasers, full color displays,infrared light detecting sensors, etc. and a process for the productionof the same.

2. Description of the Related Art

Lately, wavelength up-conversion materials utilizing the electronictransition between a plurality of energy levels of rare earth ions havebeen watched with keen interest because they can be applied to variousfields such as blue or green solid lasers, full color displays, infraredlight detecting sensors, etc. As a transparent wavelength up-conversionmaterial with a relatively high conversion efficiency, fluoride singlecrystals and glasses have hitherto been known however, it is impossibleto produce a fluoride single crystal having an optical homogeneity andpractical large size on a commercial scale and in an economical manner,since the fluoride single crystal is excellent in conversion efficiency,mechanical strength and chemical stability, but is difficult growth of ahigh quality one. On the other hand, since the fluoride glass in theform of a fiber is capable of effectively sealing up an excitation lightin a fiber core, a visible light fiber laser having the highest infraredconversion efficiency has been obtained at the present time. However,the fluoride glass is inferior to the fluoride single crystal inthermal, mechanical and chemical stability and has problems ofdurability, reliability, etc. For example, the fluoride glass tends tobe deteriorated by erosion with water and when subjected to irradiationby a laser beam with a large power, it is very liable to be damaged. Forthe preparation of a fluoride glass fiber, furthermore, precise controlof the preparation conditions such as temperature, atmosphere, etc. isindispensable, thus resulting in an increase of the production cost.

On the other hand, as a glass having a very high stability, there isknown a composition, typical of optical glasses, comprising, as apredominant component, oxides. Ordinary optical glasses containing theso-called glass-forming materials having a very high chemical bondingstrength, such as SiO₂, GeO₂, AlO₁.5, BO₁.5, PO₂.5, etc., exhibit highviscosity and are much more excellent in moldability, water resistance,mechanical strength, etc. than fluoride glasses containing ionic bondingcompounds. In oxide glasses, however, the emission efficiency from anumber of rare earth ion levels is lower by several figures than that offluorides, and the oxide glasses are not suitable for use as applyingdevices utilizing emission of rare earth ions, for example, lasermaterials.

As the wavelength up-conversion material of the prior art, the glassconsisting of high quality fluoride meets with a higher production costand the oxide glass with high stability meets with a lower emissionefficiency.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a high efficiencywavelength up-conversion transparent glass ceramics compositioncontaining rare earth ions.

It is another object of the present invention to provide a transparentmaterial having an excellent wavelength up-conversion property as wellas a high stability, whereby the problems of the prior art can besolved.

It is a further object of the present invention to provide a process forthe production of a high efficiency wave-length up-conversiontransparent glass ceramics composition containing rare earth ions.

These objects can be attained by a transparent glass ceramicscomposition having the following chemical composition, in which fluoridefine crystals containing rare earth ions are preferentiallyprecipitated:

    ______________________________________                                        SiO.sub.2    10-60 mol % (preferably 20-45 mol %)                             AlO.sub.1.5   0-40 mol % (preferably 10-30 mol %)                             GaO.sub.1.5   0-40 mol % (preferably 10-30 mol %)                             PbF.sub.2     5-60 mol % (preferably 20-50 mol %)                             CdF.sub.2     0-60 mol % (preferably 10-40 mol %)                             GeO.sub.2     0-30 mol % (preferably 0-20 mol %)                              TiO.sub.2     0-10 mol % (preferably 0-6 mol %)                               ZrO.sub.2     0-10 mol % (preferably 0-6 mol %)                               ReF.sub.3 or ReO.sub.1.5                                                                    0.05-30 mol % (preferably 0.5-20 mol %)                         (Re = Er, Tm, Ho,                                                             Yb, Pr, etc.)                                                                 ______________________________________                                    

and by a process for the production of a transparent glass ceramicscomposition, comprising preparing an oxide-fluoride glass containingrare earth ions by an ordinary melting method, subjecting the glass to aheat treatment at a temperature of higher than the glass transitiontemperature and thereby precipitating preferentially fluoride finecrystals containing a large amount of rare earth ions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the principle and merits of thepresent invention in detail.

FIG. 1 is an illustration of emission spectra when Example 1 of theinfrared-to-visible red-to-visible up-conversion material of the presentinvention is excited by an infrared light of wavelength 980 nm [(a)before heat treatment, (b) after heat treatment and (c) Er--Yb addedfluoride glass according to Comparative Example 1].

FIG. 2 is an illustration of the scattering curves of the material ofExample 1 of (a) before heat treatment and (b) after heat treatment.

FIG. 3 is a depiction of the transmission spectra of the material ofExample 1 of (a) before heat treatment and (b) after heat treatment.

FIG. 4 shows the excitation power dependence of the emission intensityof the glass ceramics and fluoride glass according to Example 1 andComparative Example 1.

FIG. 5 is visible light emission spectra when (a) the material ofExample 2 and (b) Tm³⁺ --and Yb³⁺ --added fluoride glass are excited byan infrared light of wavelength 980 nm.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have made various efforts to develop a high efficiencywavelength up-conversion transparent glass ceramics compositioncontaining rare earth ion to be applied to short wavelength solidlasers, full color displays, infrared light detecting sensors, etc. andconsequently, have found the transparent glass ceramics compositionhaving the foregoing chemical composition, in which fluoride finecrystals containing rare earth ions are preferentially precipitated.

In the foregoing chemical composition according to the presentinvention, SiO₂ and AlO₁.5 or GaO₁.5 are important as glass rawmaterials for forming the network structure of the transparent glassmatrix. With an increase of the content of SiO₂, the glass tends to beformed, but desired fluoride fine crystals are hard to be precipitated.In particular, when SiO₂ is at least 60 mol % or AlO₁.5 or GaO₁.5 is atleast 40 mol %, the fine crystals cannot be precipitated and theemission efficiency is decreased. PbF₂ and CdF₂ not only promoteformation of the glass, but also play an essential role of precipitatingthe fine crystals. When more than 60 mol % of PbF₂ and CdF₂ are added,the glass is not formed. GeO₂ is a component playing the same role asSiO₂ and even when a part of SiO₂ is replaced by GeO₂, the formation ofthe glass is not affected. TiO₂ or ZrO₂ is effective for theprecipitation of the fine crystals even if added in a small amount andwhen adding at least 10 mol %, this component is not dissolved. Rareearth elements (Re⁺³) are emission components, of which Yb⁺³ a as asensitizer is preferably used in a larger amount, e.g. about 20 mol %and other Re³⁺ elements are preferably used in an amount of up to 5 mol% so as to prevent the concentration quenching.

In the present invention, there is provided a process for the productionof a transparent glass ceramics composition, comprising preparing anoxide-fluoride glass containing rare earth ions by an ordinary meltingmethod, subjecting the glass to a heat treatment at a temperature ofhigher than the glass transition temperature and thereby precipitatingpreferentially fluoride fine crystals containing a large amount of rareearth ions. Specifically, glass-forming materials, for example, powdersof SiO₂, AlO₁.5 and fluorides such as PbF₂, CdF₂, ReF₃ (rare earthelement fluorides) are mixed, charged in a platinum crucible,solubilized in the air at a temperature of about 1000° C., cast in amold of carbon and charged in an annealing furnace to remove the strain.Up to this step, substantially the same process is carried out as in theordinary processes for the production of oxide glasses, thus obtainingtransparent glasses with various shapes. When the thus resultingtransparent glass is further heat treated for several hours at atemperature of higher than the glass transition temperature, at which nodevitrification occurs, the emission efficiency of the rare earthelements can at once be inceased without deteriorating the transparency.In such a very simple method, a transparent body with a high rare earthemission efficiency can be obtained, because a large amount of fluoridefine crystals containing rare earth ions are precipitated in the glassby the heat treatment. Since a large amount of the fine crystal grainsprecipitated in the resulting transparent glass ceramics have sizes ofsmaller than the light wavelengths, however, scattering of visible lightis so little as can be neglected and the glass apparently has acompletely same transparency before and after the heat treatment,although differing in structure. Concerning the components, moreover,GeO₂ or BO₁.5 can be used as a raw material or raw component instead ofSiO₂, GaO₁.5 or TiO₂ instead of AlO₁.5 and rare earth oxides ReO₁.5instead of the rare earth fluorides ReF₃. The important point consistsin stably incorporating fluorides into an oxide glass, subjecting themixture to a heat treatment and thereby precipitating fluoride finecrystals containing rare earth ions. Heavy metal fluorides such as PbF₂,CdF₂, TlF, etc. are preferable as the component, since they metastablyform ionic bonds with fluoride ion in the oxide glass network andfunction to prevent the rare earth ion from direct bonding with Si--O.The atmosphere used for melting the glass is not limited, but can beair. As the crucible, there can be used crucibles of platinum, alumina,silica, etc.

The above described production process will specifically be illustratedin the following. Powders of oxides and fluorides as raw materials aremixed, charged in a platinum crucible provided with a cover anduniformly melted at about 1000° C. When the content of AlO₁.5, TiO₂ orZrO₂ each having a high melting point is larger, it is preferable toadjust the melting temperature to somewhat higher (less than 1200° C.).If the temperature is higher than 200° C., however, a loss of thefluoride due to evaporation is so increased that the melt should beprevented from such a high temperature. The melting time, depending uponthe amount of one batch, is generally 1 to 2 hours in the case of about100 g, and with the increase of the amount thereof, the melting time ispreferably lengthened for homogenizing the mixture. However, themaintenance of the melt for a longer period of time should be avoidedsince the loss of the fluorides due to evaporation is remarkable.

The glass melt is cast in a mold, solidified, annealed near the glasstransition temperature (Tg), cooled therefrom to a temperature of about50° C. lower than Tg, further heated and heat-treated at a temperatureof about 100° C. higher than Tg and thus fine crystals are precipitated.Generally, Tg is different depending on the composition of the glass andaccordingly, the heat treatment temperature is different according tothe glass composition. As long as denitrification does not occur, it ispreferable to effect the heat treatment at a higher temperature from thestandpoint of efficiency and at a higher temperature, the heat treatmenttime is shorter, but in general, the heat treatment should preferably becarried out for at least 10 hours.

EXAMPLES

The present invention will now be illustrated in greater detail by thefollowing examples, but the present invention and merits thereof are notintended to be limited by the materials, compositions and productionprocedures described in these examples. Of course, these examples areonly given to exemplify the present invention and can variously bechanged or modified without departure from the scope of the presentinvention.

Example 1

Powders of SiO₂, AI(OH)₃, PbF₂, CdF₂, YbF₃ and ErF₃ were weighed andmixed to give a composition of 30% SiO₂, 15% AlO₁.5, 24% PbF₂, 20% CdF₂,10% YbF₃ and 1% Errs (mol %). The resulting mixture was charged in aplatinum crucible and melted for about 1 hour in the air at atemperature of 1050° C. The uniformly melted mixture was cast in a moldof carbon and annealed at 400° C. The thus obtained transparent glasswas further heat treated at a temperature of 470° C. for 7 hours andalowed to stand and cool at room temperature.

FIG. 1 shows the emission spectra when (a) the glass free from the heattreatment and (b) the same glass but heat-treated at 470° C. areirradiated by a semiconductor laser with a wavelength of 980 nm. Theemissions at a wavelength band of 550 nm and 660 nm are respectivelygreen and red emission by Er³⁺ and in particular, the emission of 550 nmis expected to be applied to short wavelength lasers and the like. It isapparent from FIG. 1 that the emission intensity in the visible range isimproved by at least about 100 times by the heat treatment at 470° C.

FIG. 2 shows X-ray scattering curves of the samples before and after theheat treatment in this Example. It is shown therein that the samplebefore the heat treatment (a) gives a broad curve characteristic of aglass, while the heat-treated sample (b) gives sharp peaks due to thepresence of crystals. That is, this tells that a large amount ofcrystalline materials are precipitated in the glass by the heattreatment. These peaks are due to fine crystals of fluoride solidsolutions of PbF₂, CdF₂, YbF₃, ErF₃, etc. and it is calculated from thehalf value width thereof that the size of the fine crystals is verysmall. i.e. in the figure of 20 nm.

FIG. 3 shows transmission spectra of the glass sample before and afterthe heat treatment, in which the dotted line shows the result after theheat treatment and the solid line shows that before the heat treatment.It is apparent from this figure that the transmittance is hardly changedbefore and after the heat treatment. Generally, when foreign matterssuch as crystals are precipiated in a glass, the transmission isremarkably lowered by scattering of light, but if the precipitatedcrystals are very small, e.g. smaller than the wavelength of light andthe difference in refractive index between the fine crystals and matrixglass is small, the loss of transmission due to scattering of light canbe so suppressed as can be neglected. The results are shown in FIG. 3.

FIG. 4 shows the excitation power dependence of the visible lightemission measured using the material according to Example 1 and asemiconductor laser of 980 nm as an excitation light source. All theemissions of 550 nm and 660 nm are due to the energy transition among aplurality of energy levels of rare earth ions (In this case, ⁴ I_(11/2),⁴ S_(3/2) and ⁴ F_(9/2) levels of Er³⁺ and ² F_(5/2) level of Yb³⁺),which teaches the square dependence of the excitation powercharacteristic of the wavelength up-conversion.

In FIG. 4, the symbols and abbreviations have the following meanings:

□: I₄₁₀, transparent glass ceramics before heat treatment

◯: I₅₅₀, transparent glass ceramics before heat treatment

Δ: I₆₆₀, transparent glass ceramics before heat treatment

: I₄₁₀, transparent glass ceramics after heat treatment

: I₅₅₀, transparent glass ceramics after heat treatment

: I₆₆₀, transparent glass ceramics after heat treatment

smaller : I₄₁₀, fluoride glass of the prior art

smaller : I₅₅₀, fluoride glass of the prior art

smaller : I₆₆₀, fluoride glass of the prior art

"I_(x) ": I=Intensity, x=Emission Wavelength

"Pin": P=Power (Excitation Light Power), in=input

Comparative Example 1

For comparison, a typical fluoride glass having hitherto been developed(35% AlF₃, 14% YbF₃, 1% ErF₃, 20% PbF₂, 5% MgF₂, 15% CaF₂, 10% BaF₂, mol%) was prepared using a platinum crucible in a nitrogen atmosphere. Ithas been confirmed that this glass is not stable because of containingno compound with strong bonding property, but has a highinfrared-to-visible conversion efficiency with a same level as thefluoride single crystal. In FIG. 1(c) and FIG. 4 are respectively shownthe emission spectrum of the glass when this is irradiated by asemiconductor laser of 980 nm and the excitation power dependence of theemission intensity. The green emission intensity of the fluoride glasswas about 1/2 times as large as that of the transparent glass ceramicsof Example 1 and the power dependences of both the glasses weresubstantially same. Thus, it was confirmed that the material of thepresent invention exhibited a higher efficiency over from a lowerexcitation power to a higher excitation light power than the fluorideglass.

Example 2 and Comparative Example 2

A transparent glass ceramics having a composition of 30% SiO₂, 15%AlO₁.5, 45% PbF₂, 10% YbF₃ and 0.1% TmF₃ (mol %) was prepared in ananalogous manner to Example 1. When this sample was irradiated by alaser of 980 nm, blue emission, one of the three primary colors, wasstrongly observed. This blue emission spectrum is shown in FIG. 5. Forcomparison, an emission spectrum of a Yb³⁺ --and Tm³⁺ --co-dopedfluoride glass (35% AlF₃, 15% YbF₃, 20% PbF₂, 5% MgF₂, 15% CaF₂, 10%BaF₂, 0.1% TmF₃, Comparative Example 2) by 980 nm excitation is alsoshown in FIG. 5. All the emissions in the visible range are due to Tm³⁺.As to the emission intensity of 360 nm, the glass ceramics is strongerthan the fluoride glass, but both the glasses shows substantially thesame blue emission of 480 nm. In FIG. 5, (a) shows an emission spectrumof the glass ceramics of Example 2 and (b) shows that of ComparativeExample 2.

Examples 3-6

Samples of Examples 3-6 were prepared in an analogous manner to Example1 and subjected to measurement of the infrared-to-visible wavelengthconversion performance, e.g. the excitation power dependence S_(x) ofthe visible light emission intensity when excited by a 980 nm laser,thus obtaining results as shown in Table 1 with the data of Examples 1and 2.

Advantages of the Present Invention

According to the present invention, there can readily be obtained atransparent glass ceramics with a high infrared-to-visible conversionefficiency and high chemical, mechanical and thermal stability, whichcan be applied to short wavelength solid lasers, full color displays,infrared light detecting sensors, etc.

                  TABLE 1                                                         ______________________________________                                                                 Comparative                                          Examples                 Examples                                             1         3      4      2    5    6    1     2                                ______________________________________                                        SiO.sub.2                                                                            30     35     30   30   35   30                                        AlO.sub.1.5                                                                          15     15     15   15   15   15                                        PbF.sub.2                                                                            24     39     44   45   40   40                                        CdF.sub.2                                                                            20                                                                     YbF.sub.3                                                                            10     10     10   10   10   10                                        ErF.sub.3                                                                            1      1      1                                                        TmF.sub.3                 0.1  0.1                                            HoF.sub.3                           0.1                                       S.sub.410                                                                            2.60   2.62   2.60                2.74                                 S.sub.545                                                                            1.68   1.82   1.85                1.87                                 S.sub.655                                                                            2.14   2.13   1.90                2.12                                 S.sub.393                 2.81 2.96            3.07                           S.sub.451                 2.78 2.93            2.95                           S.sub.477                 1.82 2.13            2.03                           S.sub.650                 1.82 2.14            2.03                           Emission                                                                             (A)    (A)    (A)  (B)  (B)  (C)  (C)   (B)                            Color                                                                         Emission                                                                             (D)    (D)    (D)  (D)  (D)  (D)  (D)   (D)                            Intensity                                                                     ______________________________________                                         Note:                                                                         (A): yellow green;                                                            (B): blue;                                                                     (C): green                                                                   (D): strong                                                              

What is claimed is:
 1. A transparent glass ceramics composition havingthe following chemical composition, in which fluoride fine crystalscontaining rare earth ions are precipitated:

    ______________________________________                                        SiO.sub.2     10-60 mol %                                                     AlO.sub.1.5   0-40 mol %                                                      GaO.sub.1.5   0-40 mol %                                                      PbF.sub.2     5-60 mol %                                                      CdF.sub.2     0-60 mol %                                                      GeO.sub.2     0-30 mol %                                                      TiO.sub.2     0-10 mol %                                                      ZrO.sub.2     0-10 mol %                                                      ReF.sub.3 or ReO.sub.1.5                                                                    0.05-30 mol %                                                   ______________________________________                                    

wherein, Re=Er, Tm, Ho, Yb, or Pr.
 2. The transparent glass ceramicscomposition according to claim 1, having the following chemicalcomposition:

    ______________________________________                                        SiO.sub.2           20-45   mol %                                             AlO.sub.1.5         10-30   mol %                                             GaO.sub.1.5         10-30   mol %                                             PbF.sub.2           20-50   mol %                                             CdF.sub.2           10-40   mol %                                             GeO.sub.2           0-20    mol %                                             TiO.sub.2           0-6     mol %                                             ZrO.sub.2           0-6     mol %                                             ReF.sub.3 or ReO.sub.1.5                                                                          0.5-20  mol %,                                            ______________________________________                                    

wherein Re=Er, Tm, Ho, Yb or Pr.